Understanding The I/O Ports And USB

The word “port” is used often in the computer industry, and has many different meanings depending on what technology is being referred to. In this section, we detail input/output (I/O) ports. I/O ports allow for connections to hardware. This hardware could be internal or external. The ports are associated with copper circuits and memory ranges that allow the communication of data between the CPU, RAM, and the ports themselves. Common I/O ports include USB and FireWire. In this section, we also discuss SCSI, audio connections, MIDI, and RG-6 coaxial ports. Although the most important I/O port on recent systems is the USB port, you might also encounter other ports, including legacy ports such as serial and parallel, which we speak to in this section as well.

Universal Serial Bus (USB) ports have largely replaced PS/2 (mini-DIN) mouse and keyboard, serial (COM), and parallel (LPT) ports on recent systems. Most recent desktop systems have at least four USB ports, and many systems support as many as eight or more front- and rear-mounted USB ports. Image shows the rear panel of a typical ATX system, including USB and other port types discussed in this chapter.

The following sections describe USB port types and how to add more USB ports.

USB Port Types, Speeds, and Technical Details

There are three standards types of USB ports you need to know:

  • USB 1.1
  • USB 2.0 (also called Hi-Speed USB)
  • USB 3.0 (also called SuperSpeed USB)

The standards use the same cable and connector types, which are shown in Image.

USB Types

USB cables use two types of connectors: Series A (also called Type A) and Series B (also called Type B). Series A connectors are used on USB root hubs (the USB ports in the computer) and USB external hubs to support USB devices. Series B connectors are used for devices that employ a removable USB cable, such as a USB printer or a generic (external) hub. Generally, you need a Series A–to–Series B cable to attach most devices to a USB root or external hub. Cables that are Series A–to–Series A or Series B–to–Series B are used to extend standard cables, and can cause problems if the combined length of the cables exceeds recommended distances. Adapters are available to convert Series B cables into Mini-B cables, which support the Mini-B port design used on many recent USB devices.

USB 1.1 ports run at a top speed (full-speed USB) of 12 megabits per second (Mbps), low-speed USB devices such as a mouse or a keyboard run at 1.5Mbps, and USB 2.0 (Hi-Speed USB) ports run at a top speed of 480Mbps. USB 2.0 ports are backward-compatible with USB 1.1 devices and speeds, and manage multiple USB 1.1 devices better than a USB 1.1 port does. USB 3.0 ports run at a top speed of 4,800Mbps.

USB packaging and device markings frequently use the official logos shown in Image to distinguish the different versions of USB in common use. Note that the industry is shifting from using the term “USB 2.0” to “Hi-Speed USB.”

The USB logo (left) is used for USB 1.1–compatible devices, whereas the Hi-Speed USB logo (right) is used for USB 2.0–compatible devices. Devices bearing these logos have been certified by the USB Implementer Forum, Inc.

With either version of USB, a single USB port on an add-on card or motherboard is designed to handle up to 127 devices through the use of multi port hubs and daisy-chaining hubs. Starting with Windows 98, USB devices are Plug and Play (PnP) devices that are hot swappable (can be connected and disconnected without turning off the system). The USB ports (each group of two ports is connected to a root hub) in the computer use a single IRQ and a single I/O port address, regardless of the number of physical USB ports or devices attached to those ports.

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